1. Field of the Invention
This invention relates to a valve casing for use in a butterfly valve and, more particularly, to a novel structure of a valve casing formed by press working of metal plate.
2. Description of Prior Art
A valve casing has been conventionally manufactured by casting a metal material such as cast iron or non-ferrous metal and machining a cast product, but recently as a result of advances in the field of sheet pressing, the valve casing is also manufactured by a method wherein a metal plate of iron or non-ferrous metal is formed into a valve casing by press working and a machining process is either omitted or applied at a minimum. By such a method, a large amount of valve casings are now efficiently mass-produced with their dimensional accuracy assured. Thus considerable savings in manufacturing costs as compared with the conventional method by casting and machining are realized.
Furthermore, since the weight of a valve manufactured by the latter method is much lighter, an advantage can be enjoyed when installing it on a pipe line, and since loads applied to the pipe line during operation of the valve are decreased, a further advantage is attained from the standpoint of maintenance.
Notwithstanding such advantages, a valve casing for use in a butterfly valve produced by press working has several disadvantages which are not the case for a valve produced by casting as discussed hereinafter referring to FIG. 7, which shows an ordinary butterfly valve in a fitting state.
In FIG. 7, reference numeral 1a is a valve casing comprising a cylindrical body 6a and a seal ring receiving portions 7a formed by bending both ends thereof making a right angle. Seal rings 9a are fitted in these seal ring receiving portions 7a so as to be in contact with flanges 10a, 10a of a pipe line on both sides. These flanges are provided with a plurality of circular apertures so as to connect one aperture on one side with the other on the corresponding side. The seal rings fitted in the receiving portions perform water sealing being pressed and contacted closely by clamping one aperture portion on the other by way of a bolt 28 and a nut 29. When a clamping force is applied in a horizontal direction, a stress produced thereby is concentrated on a bending line L of each seal ring receiving portion 7a, and accordingly there is the possibility of deformation, breakage or failure due to a reduction in the thickness of the metal plate. In particular, the bending line area is formed thinner than its original thickness by a rolling process using a press in order to enlarge or extend a diameter of each end portion to be larger than the cylindrical body 6a, otherwise a seal ring receiving member formed separately is fixed to this area. It is to be understood that this area is lowest in strength of all parts of the valve casing, and as a result the problem of deformation or breakage occurs easily at this area.
Several attempts have been proposed in order to solve the above-discussed problems. For example, as shown in FIG. 8, a plurality of reinforcing ribs 30 like triangular pieces are fitted by welding to a corner where the cylindrical body 6b is adjacent to the seal ring receiving portion 7b. Alternatively as shown in FIG. 9, a cylindrical body 6c is spread out or diverged from a middle part thereof (Japanese laid open Utility model Publication No. 60-8577 titled "Structure of Valve Casing" disclosed by the applicant).
Both of these prior attempts, however, have the following disadvantages.
In the case of the former (FIG. 8), since variance is produced as a result of resistance to inner pressure, depending on their portions, a large number of ribs are required to the extent that production efficiency is lowered, thus the mass-production thereof is inadequate.
In the case of the latter (FIG. 9), an advantage is attained as a result of resistance to compressive stress since a load applied thereto is divided in the form of a vector. However, since a load applied by fluid pressure to a periphery of a pipe is increased in proportion to the diameter of the pipe, a large amount of load is applied to an inner wall of the spread out portion. Accordingly, it is necessary to calculate a thickness of the metal plate for resistant to such an amount of load, as well as allowance for reduction in thickness, when deciding on thickness considering that the thickness of the metal plate is decreased as the amount of spreading or enlargement is increased, in the process of press working.
A further disadvantage of the valve casing formed by press working exists in that the seal ring easily gets out of place or is broken. Referring to a section 7a or 7c of each seal ring receiving portion shown in FIGS. 7 and 9, since the width Wa.sub.1 or Wc.sub.1 of the outer diameter side of the seal ring to be received is formed either the same or a little smaller than the width Wa.sub.2 or Wc.sub.2 of the inner diameter side thereof, the seal ring may be sucked inward by fluid passing through a bore of the valve body, thereby creating the problem of imperfect sealing or breakage of the seal ring. As a problem common in valve casings in general (including one made of cast iron besides one formed by press working), a seal ring inevitably deteriorates by aging with the lapse of time causing thereby lowering in the sealing performance of the valve casing. It becomes necessary to replace the aged seal ring with a new one or to clamp the seal with a clamping bolt to reinforce the lowered sealing performance.
There is also the possibility that when tight fitting of the seal ring is affected due to aging, and a space is produced between the seal ring and the seal ring receiving portion, corrosion eventually occurs therebetween, depending on the material, which results in shortening the life span of the valve. As is well known, such a corrosion problem occurs easily when austenitic stainless steel material is used.
Furthermore, when the temperature or chemical atmosphere is different between the inside and the outside of a pipe line, there still exists a problem of selection of the valve structure and material in view of fitting a valve to the pipe line.
For example, when the temperature of the fluid passing through a bore of a valve is high while outside air temperature is low, energy loss due to heat radiation is increased, and when a temperature of fluid is ordinary while outside air temperature is low, the problem of freezing, or break down of the pipe line may occur.
As for the chemical atmosphere, the selection of material is a difficult problem particularly when the outside atmosphere of the pipe line is alkaline.